1. Technical Field
This invention relates to the evaporative casting process (ECP) employing a mold within which is embedded a relatively low density pattern material that is destroyed and evaporated upon contact with molten metal poured into the mold. More particularly, this invention relates to the art of designing and making such patterns for improved accuracy of dimensions in the making of metal castings.
2. Description of the Prior Art
ECP employs a heat consummable pattern and associated cast metal feeding members (hereinafter low density system) that are all commonly made of the same material. The material is preferably low density foam expanded bead polystyrene. Upon contact with molten metal poured into the mold containing such low density system, the material is ignited, burned and evaporated, and the gases therefrom migrate outwardly through the interstices of unbonded sand forming the mold.
In the process of settling dry, unbonded sand about such low density system, the patterns may become slightly misaligned or distorted by the sand filling forces. It has become common practice to attach several of such patterns to a central, consummable, upright sprue by way of radiating horizontal runners (see U.S. Pat. No. 4,003,424). Complex patterns which have one or more projections from such pattern are subject to distortion by the filling forces of such sand as the sand advances along an angle of repose during the filling operation. The projections of such patterns, as well as the pattern itself, are frequently cantilevered structures, that is, they are hung at the end of a horizontally extending member.
Although no prior art has been found which addresses itself to the problem mentioned above, within the art of ECP, an examination of the lost wax method of casting discloses that multiple patterns have been hung on trees along with a gating system (see U.S. Pat. Nos. 4,109,699; 4,061,175; and 4,064,927). All of these disclosures describe the use of compact patterns (in the form of puck-shaped cylinders). Such patterns and associated gating systems are not distorted under the lost wax method since the wax pattern is applied by dipping a skeletal structure in a molten wax medium and then into a ceramic slurry. The lost wax coatings, when solidified, are stronger than low density polystyrene and the coating configuration is never subjected to surrounding distortion forces once solidified. The ceramic slurry is applied as a light dipped coating in a fluidized condition with little or no forces on the solidified wax coatings. Third, the problem of pattern distorting forces from sand fillings does not appear in the lost wax art.
What is needed is a method and system design that enables distortable patterns to be rigidified in location without an undesirable increase of gating that results in increasing scrap metal.